Cryo-EM single particle analysis is a method for determining structures of large molecules and macromolecular assemblies at resolutions ranging from 3.5 - 30 A. Interpreting the density maps produced by this technique represents an ongoing challenge, for which molecular modeling techniques offer some unique solutions.

Over the last five years, cryo-EM single particle analysis has begun producing structures at resolutions better than 5 A, with subnanometer resolutions becoming common. At resolutions between 5 and 9 A it becomes possible to move beyond simple rigid-body docking and alter atomistic models to reposition helices and sheets, to better fit the cryo-EM based density maps. At 3-5 A resolution de-novo C-alpha traces and in some cases full atomistic models can be constructed directly from the cyro-EM density without invoking x-ray crystallography.

We call this a challenge rather than a contest because, unlike CASP, there is no hidden answer to be revealed. In this project, we provide publicly available cryo-EM densities for a selected set of structures at different resolutions, and challenge those in the modeling community to apply their tools to extract as much information as they can from each. At the end, the results will be evaluated by comparing the results of different groups, and validating against any other existing knowledge about each target. We hope this will yield new insights into these published structures, and at the very least, it will establish the capabilities of current modeling methods, and give the cryo-EM community some guidance as to how to proceed with maps in various resolution ranges. For modelers it provides a new area in which to apply/develop their techniques, and demonstrating your tools' capabilities may lead to new opportunities for collaboration.